Mr Glassey said the bone-chilling -20 degrees recorded at Pukaki was close to the country's coldest ever recorded of -25.6 in Ranfurly in 1903.

"So that's very cold."

Dunedin Airport and Blenheim were -5 degrees, while Dunedin city was just below 0 degrees, he said.

Taumarunui was the coldest town in the North Island with -5 degrees, followed by Taupo on -3 and Hamilton -2.

Auckland International Airport was 2.7 degrees while Ardmore Airport was currently sitting at 1.5 degrees.

Meanwhile, Canterbury Police have issued a black ice warning and urging motorists to take extreme care this morning.

Senior Sergeant Richard Ryan said the condition of the district's roads were "treacherous as the overnight rain has now frozen".

"Police have attended a couple of minor accidents already today that can be attributed to the black ice which has set on the roads. Road contractors are being utilised to try and alleviate the current problem."

Police ask motorists to allow extra time for travel today, to drive to the conditions and be mindful that black ice is often very difficult to see.

In Christchurch, University of Canterbury exams have been pushed back an hour due to the big freeze.

"Due to icy road conditions in Christchurch a decision has been made to delay the start of morning examinations until 10am," a campus spokesperson said via the university's Facebook page.

"Afternoon examinations will go ahead as scheduled. Students are urged to take care travelling to and from university."

Coldest parts of NZ right now:

-20 degrees recorded at Pukaki, close to the country's coldest ever recorded of -25.6 in Ranfurly in 1903